Vapor barriers, sometimes known as vapor retarders,
are essential in most environments in controlling moisture in your home. When
excessive moisture builds up it can cause severe damage to the homes
structure, causing the wood to rot and can also ruin the insulation.

The most common vapor barriers used are plastic
sheeting (visqueen), kraft paper adhered to fiberglass insulation and
reflective foil sheeting. The materials resistance to moisture is measured by
its permeance value, also know as a perm rating.

In most climates, the most important areas of
the home that will require a vapor retarder are the exterior walls, crawlspace
and ceiling (when insulating with fiberglass batts).